Boreholes drilled into the ground are rarely straight, owing to the interaction of a rotating drill bit with inhomogeneous ground. Nevertheless, deviation of the borehole is intentional in some instances. To determine the true trajectory, borehole survey instruments are run through the hole. Measurements of dip (or inclination) and azimuth can be made at intervals along the borehole and the results interpolated to derive the trajectory. Sometimes the measurements are made while moving the instrument along the borehole; sometimes the measurements are made at discrete stationary intervals.
Sensing inclination is relatively straightforward. There is considerable prior art related to accelerometers and tiltmeters that can be used to determine the inclination of the instrument axis with the gravitational vertical. Sensing azimuth is more difficult. There are three methods of doing this: (1) using a sensor referenced to an inertial frame of reference (i.e., a gyroscope), (2) using the Earth's magnetic field (i.e., a magnetometer or compass), or (3) integrating changes in curvature of the borehole from an initial surface orientation.
Current gyroscopically-oriented instruments are the most complex and expensive. Typically, they rely on one or two rotating-mass gyroscopes in a gimballed or strap-down mount. As the instrument is run through the borehole, movements of the axis (in the case of gimballed gyroscope) or precessive forces (in the case of a strap-down gyroscope) are measured to obtain the attitude of the instrument. Unfortunately, rotating gyroscopes are sensitive to shock loading, drift, and external magnetic fields. Much of the effort in improving the accuracy of gyroscopically-oriented revolves around compensating for drift in the gyroscopes.
Instruments using the Earth's magnetic field are of relatively simple construction and are therefore the least expensive. Unfortunately, magnetic fields generated by the drilling equipment and/or nearby mineralization can distort the local magnetic field, and hence the reported azimuth. Instruments with three-component magnetometers (e.g., a fluxgate magnetometer) are able to give some diagnostic information such as magnetic field strength and magnetic dip. These diagnostic parameters should be constant for a given geographical location and any significant variation can be taken as an indication that the reported azimuth is less reliable. Additionally, magnetically-oriented survey instruments do not operate well at high geomagnetic latitudes because the Earth's magnetic field is near-vertical.
Integration-type instruments are typically run inside the drill rods. A snug fit between the instrument and the rods ensures that the instrument takes on the same curvature as the rods. As the instrument is run through the rods, the twists and turns of the rods are added up to obtain the borehole trajectory. Unfortunately, measurement errors accumulate and the error of these instruments increases with hole depth.